Earliest evidence of animals yet?

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Scientists say they have found fossil evidence of an "animal-like" creature which they report is twice as old as any animal fossils generally accepted by palaeontologists.

A paper by Birger Rasmussen of the University of Western Australia and colleagues published in this week's issue of Science provides dates and a detailed description of samples originally discovered 10 years ago in sandstone of the Stirling Range Formation in south-western Australia.

The palaeontologist on the team interpreted the well-preserved fine ridges left in the sandstone as being slime tracks of a mucus-producing worm-like creature creeping over the surface of the sand.

Dating of the sediments indicate they are more than 1.2 billion years old.

According to co-author Ian R. Fletcher, the Stirling Range sandstone was first dated 25 years ago at 1.3 billion years. However when Rasmussen and colleagues discovered the fossils 10 years ago, they noted their resemblance to a well-studied assemblage of creatures known as the Ediacaran biota, which are younger than 600 million years old.

This most recent evidence, however, has confirmed that the rocks are much older and if the marks in the rock really are fossils then it is the best evidence yet of animal-like organisms of this age.

"We can expect a lot of scepticism," says Fletcher. "However we have considered all the alternative explanations we can think of."

Previous claims for animal-like fossils of this age have met with controversy. Collecting evidence to argue the case one way or the other is very difficult since evidence is scarce.

Firstly, there are very few fossils older than 600 million years and certainly no bones and teeth left to confirm them.

Secondly, any fossils that do exist are mostly found in simple sandy sediments, the ages of which are notoriously difficult to determine.

The same team has been working on new, more reliable methods of dating sedimentary rocks that rely on the decay of radioactive uranium and thorium in minerals.

In 1999, the team announced a dating method based on radioactive elements in a mineral called xenotime. For the Stirling Range material they've used another mineral, monazite.

The researchers say that their findings are consistent with some of the calculations made by molecular biologists as to when animals first appeared on Earth.

However more research is needed.

"That's why we prefer to say the fossils are "animal-like", rather than of animals per se" says Rasmussen. "It might look like a duck and walk like a duck. We just don't know if it actually quacked."